Plug, retaining member, and method for well completion using plug

ABSTRACT

A plug which does not cause an adverse effect such as failure of a member constituting the plug when a wellbore is plugged. A downhole plug (10) includes a mandrel (1) having a hollow structure, an annular sealing member (2) attached to an outer circumferential surface of the mandrel (1) and being deformable by exerted pressure, and an annular socket (3) attached, adjacent to the annular sealing member (2), onto the outer circumferential surface of the mandrel (1), downstream of where a pressure is exerted on the sealing member (2). The annular socket (3) is configured to include a socket-inner portion (31) which comes into contact with the outer circumferential surface of the mandrel (1), and a socket-outer portion (32) in an annular shape having an inner diameter equivalent to or larger than that of the socket-inner portion (31) and movably attached to the socket-inner portion (31). The socket-outer portion (32) includes a face facing the sealing member (2).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of copending application Ser. No.16/617,282, filed on Nov. 26, 2019, which is the National Phase under 35U.S.C. § 371 of International Application No. PCT/JP2018/027526 filed onJul. 23, 2018, and claims priority under 35 U.S.C. § 119(a) to PatentApplication No. 2017-156159, filed in Japan on Aug. 10, 2017, all ofwhich are hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present invention relates to a plug and a retaining member used forwell completion, and a method for well completion using the plug.

BACKGROUND ART

Various tools have been developed which are referred to as downholetools for plugging and fixing a wellbore for excavating shale oil byhydraulic fracturing or the like. As one of these downhole tools,downhole plugs are known. One of the functions of the downhole plug isthat a prescribed member constituting the downhole plug abuts against aninner wall of a wellbore to fix the downhole plug to the wellbore, andthe wellbore is blocked by an elastic member or the like constitutingthe downhole plug (for example, Patent Document 1).

CITATION LIST Patent Document

Patent Document 1: US 2011/0,277,989 A (published on Nov. 17, 2011)

SUMMARY OF INVENTION Technical Problem

Incidentally, in case of a downhole tool with which a wellbore isplugged by an elastic member, supposing that deformation of the elasticmember is not controlled, a desired advantage cannot be obtained due toan unanticipated deformation of the elastic member, where, in the worstcase, an issue arises in that a breakage is caused to other componentsconstituting the downhole plug. The above issue will be described belowin detail with reference to FIGS. 6 to 7B.

FIGS. 6 to 7B are explanatory reference views illustrating an issue indownhole plugs in the related art. FIG. 6 is a view schematicallyillustrating a portion of an axial cross-section of the downhole plug inthe related art. FIGS. 7A and 7B are views illustrating a portion of thedownhole plug illustrated in FIG. 6. Note that, for convenience ofexplanation, in FIGS. 6 to 7B, an axial direction of the downhole plugis illustrated as the horizontal direction of the plane of the paper. Inan actual use, however, the downhole plug is occasionally disposed suchthat the axial direction of the downhole plug extends along the depthdirection of a wellbore. In addition, in FIGS. 7A and 7B, shapes of somemembers illustrated in FIG. 6 are illustrated in a simplified manner.

First, as illustrated in FIG. 6, a downhole plug 100 includes a mandrel101, an elastic member 102, a retaining member 103 disposed adjacent tothe elastic member 102 on one side of the elastic member 102, cones 104and 105 disposed so as to sandwich the elastic member 102 and theretaining member 103, a pair of slips 106 a and 106 b, and a pair ofring members 107 a and 107 b.

In a wellbore (not illustrated), the downhole plug 100 is installedwithin a casing 20 disposed inside the wellbore as illustrated in FIG.7A. When the wellbore is plugged by the downhole plug 100, the elasticmember 102 is deformed as a gap between the cone 105 and the retainingmember 103 is narrowed due to an axial movement of the mandrel 101.Then, the elastic member 102 extends radially outward in acircumferential direction of an axis of the mandrel 101. Then, theelastic member 102 abuts against the casing 20, to thus cause betweenthe downhole plug 100 and the casing 20 to be plugged. Subsequently, thewellbore is plugged by setting a ball or the like at a hollow portion inan axial direction of the mandrel 101. A fluid is then pumped at highpressure into the plugged section from the cone 105 side, to performhydraulic fracturing for forming fractures in a productive layer.

Unfortunately, in case when the deformation of the elastic member 102 isnot controlled, the elastic member 102 enters between the mandrel 101and the retaining member 103 as indicated by an arrow in FIG. 7B whenthe elastic member 102 is deformed by a pressure exerted on the cone105. This allows the elastic member 102 having entered between themandrel 101 and the retaining member 103 to constrict or extend themandrel 101. As a result, deformation may occur in the mandrel 101, or abreakage may be caused to other components (not illustrated)constituting the downhole plug.

Note that the retaining member 103 described above is occasionallyintegrally formed with the cone 104. The downhole plug having such aconfiguration will be described below with reference to FIG. 8.

FIG. 8 is a view illustrating another aspect of a downhole plug in therelated art, and schematically illustrating a portion of an axialcross-section of the downhole plug. Note that, for convenience ofexplanation, members having the same function as those illustrated inFIG. 6 are denoted by the same reference signs, and descriptions ofthese will not be given.

As illustrated in FIG. 8, in a downhole plug 200, a retaining member 203is formed as an integrated component formed by integrating the retainingmember 103 and the cone 104 illustrated in FIG. 6. Even with thedownhole plug 200 thus configured, an issue similar to that in thedownhole plug 100 described above (see FIG. 6) arises.

The present invention has been made in view of the above-describedissues, and aims to provide a plug for well completion, which does notcause an adverse effect such as failure of a member constituting theplug when a wellbore is plugged.

Solution to Problem

In order to resolve the above-described issues, a plug according to anaspect of the present invention is a plug configured to plug a wellboreprovided at a well during completion, the plug including:

a tubular member having a hollow shape;

an elastic member having an annular shape attached to an outercircumferential surface of the tubular member, the annular elasticmember being deformable by exerted pressure; and

a retaining member having an annular shape attached, adjacent to theelastic member, onto the outer circumferential surface of the tubularmember, downstream of where a pressure is exerted on the elastic member,wherein

the retaining member is configured to include

an annular inner portion which comes in contact with the outercircumferential surface of the tubular member, and

an annular outer portion having an inner diameter that is equivalent toor larger than the annular inner portion and being movably attached tothe annular inner portion,

the annular outer portion including a face facing the elastic member.

Another aspect of the present invention provides a retaining member usedfor a plug provided at a well during completion and having a function ofplugging a wellbore. A retaining member according to an aspect of thepresent invention is a retaining member having an annular shape, usedfor a plug configured to plug a wellbore provided at a well duringcompletion,

the plug including:

a tubular member having a hollow structure; and

an elastic member with an annular shape attached to an outercircumferential surface of the tubular member, the annular elasticmember being deformable by exerted pressure, wherein

the retaining member is attached, adjacent to the elastic member, ontothe outer circumferential surface of the tubular member, downstream ofwhere a pressure is exerted on the elastic member,

the retaining member being configured to include

an annular inner portion which comes into contact with the outercircumferential surface of the tubular member, and

an annular outer portion having an inner diameter that is equivalent toor larger than the annular inner portion and being movably attached tothe annular inner portion,

the annular outer portion including a face that is to face the elasticmember.

Still another aspect of the present invention provides a method for wellcompletion. The method for well completion according to the stillanother aspect of the present invention uses a plug according to anaspect of the present invention.

Advantageous Effects of Invention

According to an aspect of the present invention, a plug can be providedwhich does not cause an adverse effect such as failure of a memberconstituting the plug when a wellbore is plugged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically illustrating a portion of an axialcross-section of a downhole plug according to an embodiment of thepresent invention.

FIG. 2 is cross-sectional perspective views of a socket-inner portionand a socket-outer portion according to an embodiment of the presentinvention.

FIG. 3 is a view schematically illustrating how a robber is deformed ina downhole plug according to an embodiment of the present invention,where FIG. 3A illustrates a view before exerting a pressure, and FIG. 3Billustrates a view after a pressure is exerted.

FIG. 4 is a view schematically illustrating a portion of an axialcross-section of a downhole plug according to a first alternative aspectof the present invention.

FIG. 5 is a view schematically illustrating a portion of an axialcross-section of a downhole plug according to a second alternativeaspect of the present invention.

FIG. 6 is a view schematically illustrating a portion of an axialcross-section of a downhole plug in the related art.

FIGS. 7A and 7B are view schematically illustrating a portion of adownhole plug illustrated in FIG. 6, where FIG. 7A illustrates a viewbefore exerting a pressure, and FIG. 7B illustrates a view after apressure is exerted.

FIG. 8 is a view of another aspect of a downhole plug in the relatedart, schematically illustrating a portion of an axial cross-section ofthe downhole plug.

DESCRIPTION OF EMBODIMENTS Downhole Plug

First, an embodiment of a downhole plug (plug) and a socket (retainingmember) according to an aspect of the present invention will bedescribed below with reference to figures.

FIG. 1 is a view schematically illustrating a portion of an axialcross-section of the downhole plug according to the embodiment. Asillustrated in FIG. 1, a downhole plug 10 includes a mandrel 1 (tubularmember), a sealing member (elastic member) 2, a socket (retainingmember) 3, cones 4 and 5, a pair of slips 6 a and 6 b, and a pair ofring-shaped fixing members 7 a and 7 b.

Note that, for convenience of explanation, in FIG. 1, an axial directionof the downhole plug is illustrated as the horizontal direction of theplane of the paper. In an actual use, however, the downhole plug isoccasionally disposed such that the axial direction of the downhole plugextends along the depth direction of a wellbore.

The downhole plug 10 is a tool for well completion used for plugging andfixing a wellbore (not illustrated).

The mandrel 1, which is a member for ensuring a strength of the downholeplug 10, has a hollow shape.

A sealing member 2, which is an annular rubber member, is attached ontoan outer circumferential surface in an axial direction of the mandrel 1between a socket 3 and a cone 5. The sealing member 2 is deformed when apressure is exerted on the downhole plug 10. It is preferred that thesealing member 2 be formed from a material that does not lose a functionof plugging a wellbore due to the sealing member 2 even in anenvironment under high temperature and high pressure, for example.Preferred materials for forming the sealing member 2 include nitrilerubber, hydrogenated nitrile rubber, acrylic rubber, and fluororubber.For the materials for forming the sealing member 2, a degradable rubbercan also be used, such as aliphatic polyester-based rubber, polyurethanerubber, natural rubber, polyisoprene, acrylic rubber, aliphaticpolyester rubber, polyester-based thermoplastic elastomer, or polyamide-based thermoplastic elastomer.

The socket 3 is an annular member attached, adjacent to the sealingmember 2, onto the outer circumferential surface in the axial directionof the mandrel 1, downstream of a pressure exerted on the sealing member2.

In the embodiment, the socket 3 includes a socket-inner portion 31 and asocket-outer portion 32 that are in an annular shape. The socket-outerportion 32 is movably attached to the socket-inner portion 31. Note thatdetails of the socket 3 will be described later.

The cones 4 and 5 are formed such that when a pressure is exerted on thepair of slips 6 a and 6 b toward the sealing member 2, the slips 6 a and6 b are slidably moved on inclined surfaces of the cones 4 and 5,respectively.

The fixing members 7 a and 7 b are annular members that fix positions ofthe slips 6 a and 6 b with respect to the axial direction of the mandrel1. The fixing members 7 a and 7 b are arranged adjacent to the slips 6 aand 6 b on the outer circumferential surface of the mandrel 1.

In the embodiment, examples of the material that forms the mandrel 1,the socket 3, the cones 4 and 5, the pair of slips 6 a and 6 b, and thepair of fixing members 7 a and 7 b include, for example, metal materialssuch as aluminum, steel, and stainless steel; fibers; woods; compositematerials; resins; and the like. The mandrel 1 can be composed of acomposite material containing a reinforcing material such as carbonfiber, specifically, for example, a composite material containing apolymerized material such as epoxy resin and phenol resin, and the like.It is preferred that the mandrel 1, the socket 3, the cones 4 and 5, thepair of slips 6 a and 6 b, and the pair of fixing members 7 a and 7 b beeach composed of a degradable resin or a degradable metal. Thisfacilitates removal of the downhole plug 10 after a wellbore processingis performed using the downhole plug 10.

Note that in this specification, the term “degradable resin ordegradable metal” refers to resin or metal that can be biodegraded orhydrolyzed, dissolved in water or hydrocarbons within a well, and thatcan further be decomposed and embrittled to be easily disintegrated bysome chemical method. Examples of the degradable resin includehydroxycarboxylic acid-based aliphatic polyesters such as polylacticacid (PLA) and polyglycolic acid (PGA); lactone-based aliphaticpolyesters such as poly-ε-caprolactone (PCL); diol-dicarboxylicacid-based aliphatic polyesters such as polyethylene succinate andpolybutylene succinate; copolymers of these, including, for example,poly(lactic-co-glycolic acid); as well as mixtures of these; and thelike. Another example is an aliphatic polyester used by combiningpolyethylene adipate/terephthalate or similar aromatic components.

Further, examples of the water-soluble resin include polyvinyl alcohol;polyvinyl butyral; polyvinyl formal; polyacrylamide (optionallyN,N-substituted); polyacrylic acid; and polymethacrylic acid. Anotherexample is copolymers of monomers that form these resins, for example,ethylene-vinyl alcohol copolymers (EVOH), acrylamide-acrylicacid-methacrylic acid interpolymers, and the like.

Examples of the degradable metal include alloys containing magnesium,aluminum, calcium, and the like as main components, for example.

Next, a configuration of the socket 3 illustrated in FIG. 1 is describedin detail with reference to FIGS. 2 and 3 in addition to FIG. 1.

FIG. 2 is cross-sectional perspective views of the socket-inner portion31 and the socket-outer portion 32 illustrated in FIG. 1. FIG. 3 is aview schematically illustrating how the sealing member 2 is deformed inthe downhole plug 10 illustrated in FIG. 1. For convenience ofexplanation, in FIGS. 2 and 3, an axial direction of the downhole plugis illustrated as the horizontal direction of the plane of the paper. Inan actual use, however, the downhole plug is occasionally disposed suchthat the axial direction of the downhole plug extends along the depthdirection of a wellbore.

Referring to FIGS. 2 and 3, the socket-outer portion 32 is movablyattached to the socket-inner portion 31 as described above. Morespecifically, in the axial direction and on a side of the sealing member2 of the socket-inner portion 31, the socket-outer portion 32 is movablyattached to the socket-inner portion 31. In the embodiment, thesocket-outer portion 32 is disposed between the socket-inner portion 31and the sealing member 2, to thus provide a configuration in which thesocket-outer portion 32 is attached to the socket-inner portion 31. Thisallows the socket-outer portion 32 to include a face 322 facing thesealing member 2.

The socket-outer portion 32 has a larger inner diameter than thesocket-inner portion 31. Accordingly, in the socket 3, only thesocket-inner portion 31 is in contact with the mandrel 1, and thesocket-outer portion 32 is not in contact with the mandrel 1. On theother hand, an outer diameter of the socket-inner portion 31 is the sameas an outer diameter of the socket-outer portion 32.

Thus, in a state illustrated in FIG. 3A, when the sealing member 2receives a pressure from the cone 5 to be deformed, the socket 3attached downstream of the pressure receives the deformation of thesealing member 2. Then, when the sealing member 2 is pressed against theface 322 of the socket-outer portion 32 that faces the sealing member 2to exert a force, the socket-outer portion 32 is deformed to extend adiameter of the socket-outer portion 32, to cause the socket-outerportion 32 to be slidably moved on the socket-inner portion 31 to changea form of the socket 3. This allows the force exerted on the socket 3from the sealing member 2 to disperse, making it possible to preventbreakage to members such as the socket 3, which constitute the downholeplug.

In view of the above, it is preferred that the socket-outer portion 32contain a material that can change to extend in diameter when a pressureis exerted on the socket-outer portion 32 from the sealing member 2. Inthe embodiment, the socket-outer portion 32 is composed of PGA.

In the downhole plug 10 of the embodiment, an inner circumferential edge311 of the socket-inner portion 31 that comes in contact with themandrel 1 protrudes toward the sealing member 2. More specifically, atip end of the protruding portion protrudes toward the sealing member 2in a manner to come into contact with the mandrel 1. That is, thedownhole plug 10 has a configuration in which the entire of the innercircumferential surface of the socket-inner portion 31, including theprotruding portion, comes in contact with the mandrel 1. Thiseffectively prevents a portion of the sealing member 2 having beendeformed by a pressure exerted in a direction toward the socket 3 fromentering between the socket-inner portion 31 and the mandrel 1.Accordingly, an occurrence of deformation or breakage of the mandrel 1,and further of other components constituting the downhole plug 10 can beprevented, due to the sealing member 2 entering between the socket 3 andthe mandrel 1.

Moreover, in the downhole plug 10 of the embodiment, a side of thesealing member 2 that comes in contact with the socket 3 enters into aconcave portion between the inner circumferential edge 311 and an outercircumferential edge 321 formed by the protrusions of the innercircumferential edge 311 of the socket-inner portion 31 and the outercircumferential edge 321 of the socket-outer portion 32. This allows,when the sealing member 2 is deformed by exerted load inside thewellbore (not illustrated), the sealing member 2 to be pressed againstthe socket 3 with less load. Accordingly, the wellbore can be pluggedwith less load.

Note that, in the downhole plug 10 according to the above-describedembodiment, the outer diameter of the socket-inner portion 31 is, butnot limited to, the same as the outer diameter of the socket-outerportion 32. For example, the downhole plug 10 may include thesocket-outer portion 32 having an outer diameter that is greater thanthat of the socket-inner portion 31. In this case, an outercircumferential surface of the socket 3 is formed only by thesocket-outer portion 32. Further, the socket-outer portion 32 may havean outer diameter that is less than the outer diameter of thesocket-inner portion 31. In this case, the outer circumferential surfaceof the socket 3 is formed only by the socket-inner portion 31.

As still another aspect, at least one of the socket-inner portion 31 andthe socket-outer portion 32 may be a component that is constituted bytwo or more components. That is, the socket 3 may be a component that isconstituted by three or more components.

As still another aspect, the socket 3 may be provided to come in contactwith the sealing member 2 even upstream of where the pressure isexerted. That is, a configuration may be employed in which the socket 3is also provided between the cone 5 and the sealing member 2, and thesealing member 2 is disposed between the two sockets.

In the embodiment, a configuration of the downhole plug is, but notlimited to, the configuration of the downhole plug 10 described above.Accordingly, in the downhole plug according to the present embodiment, anon-illustrated socket-outer may have an inner diameter commensuratewith that of the socket-inner, and may be movably attached to thesocket-inner.

The downhole plug according to the present embodiment may further beimplemented as a first or second alternative aspect that will bedescribed below.

First Alternative Aspect

In a first alternative aspect of the downhole plug according to theembodiment, the socket-inner 31 and the cone 4 in the downhole plug 10illustrated in FIG. 1 may be integrated with each other. A downhole plughaving such a configuration will be described below with reference toFIG. 4.

FIG. 4 is a view schematically illustrating a portion of an axialcross-section of a downhole plug according to the first alternativeaspect. For convenience of explanation, in FIG. 4, an axial direction ofthe downhole plug is illustrated as the horizontal direction of theplane of the paper. In an actual use, however, the downhole plug isoccasionally disposed such that the axial direction of the downhole plugextends along the depth direction of a wellbore. Note that, forconvenience of explanation, members having the same function as thoseillustrated in FIG. 1 are denoted by the same reference signs, anddescriptions of these will not be given.

As illustrated in FIG. 4, in a downhole plug 53, a socket-inner 33 isformed as an integrated component that the socket-inner 31 and the cone4 illustrated in FIG. 1 are integrated with each other, as describedabove. That is, a configuration is employed in which the cone alsoserves as a socket, specifically, a socket-inner. Thus, a structure onthe side of the sealing member 2 of the inner circumferential edge ofthe cone includes the structure on the side of the sealing member 2 ofthe inner circumferential edge of the above-described socket. As thedownhole plug 53 thus configured, the same advantage is obtained as inthe downhole plug 1 (see FIG. 1).

Second Alternative Aspect

In a second alternative aspect of the downhole plug according to thepresent embodiment, the inner diameter of the socket-outer may be thesame as the inner diameter of the socket-inner. A downhole plug havingsuch a configuration will be described below with reference to FIG. 5.

FIG. 5 is a view schematically illustrating a portion of an axialcross-section of a downhole plug according to the second alternativeaspect. For convenience of explanation, in FIG. 5, an axial direction ofthe downhole plug is illustrated as the horizontal direction of theplane of the paper. In an actual use, however, the downhole plug isoccasionally disposed such that the axial direction of the downhole plugextends along the depth direction of a wellbore. Note that, forconvenience of explanation, members having the same function as thoseillustrated in FIG. 1 are denoted by the same reference signs, anddescriptions of these will not be given.

As illustrated in FIG. 5, in a downhole plug 63, the inner diameter of asocket-outer 34 is the same as the inner diameter of a socket-inner 35.The socket-outer 34 has the outer diameter larger than the socket-inner35, and the socket-inner 35 is disposed at a position at which thesocket-inner 35 is disposed between the socket-outer 34 and the sealingmember 2. As the downhole plug 63 thus configured, the same advantage isobtained as in the downhole plug 1.

Note that, in the downhole plug of the second alternative aspect, thesocket-outer may be integrally formed with a cone. That is, aconfiguration may be employed in which the cone also serves as a socket,specifically, a socket-outer.

Method for Well Completion

The method for well completion according to an embodiment of the presentinvention performs well completion using the downhole plug 10 describedabove. The above method can be similar to the method of well completionin the related art using a downhole plug except for using the downholeplug 10 as a downhole plug.

According to Embodiment, when a wellbore is plugged by the sealingmember 2, a deformation of the sealing member 2 does not cause anadverse effect such as a breakage on members constituting the downholeplug. Thus, a well completion can be performed in an efficient manner.

SUMMARY

A plug according to an aspect of the present invention is a plugconfigured to plug a wellbore provided at a well during completion, theplug including:

a tubular member having a hollow shape;

an elastic member having an annular shape attached to an outercircumferential surface of the tubular member, the annular elasticmember being deformable by exerted pressure; and

a retaining member having an annular shape attached, adjacent to theelastic member, onto the outer circumferential surface of the tubularmember, downstream of where a pressure is exerted on the elastic member,wherein

the retaining member is configured to include

an annular inner portion which comes in contact with the outercircumferential surface of the tubular member, and

an annular outer portion having an inner diameter that is equivalent toor larger than the annular inner portion and being movably attached tothe annular inner portion,

the annular outer portion including a face facing the elastic member.

According to the above-described configuration, an elastic member and aretaining member adjacent to the elastic member are provided at theouter circumferential surface of the tubular member having a hollowstructure. The retaining member is located downstream of where thepressure is exerted on the elastic member. Thus, when the elastic memberis deformed by a pressure exerted inside the wellbore, the retainingmember attached downstream of the pressure is to receive thedeformation. Here, the retaining member is configured to include anannular inner portion and an annular outer portion, where the annularouter portion is movably attached to the annular inner portion. As aresult, the elastic member presses against a face of the annular outerportion facing the elastic member to exert a force, the annular outerportion moves to change a form of the retaining member. This allows aforce exerted on the retaining member from the elastic member todisperse, making it possible to prevent breakage to the retainingmember.

Further, in the plug according to an aspect of the present invention, itis preferred that an inner circumferential edge of the plug that comesin contact with the tubular member in the annular inner portion protrudetoward the elastic member.

The above-described configuration can prevent a portion of the elasticmember having been deformed by a pressure exerted in a direction towardthe retaining member from entering between the inner portion of theretaining member and the tubular member. This prevents deformation orbreakage of the tubular member from occurring, due to the elastic memberentering between the retaining member and the tubular member.

Further, in the plug according to an aspect of the present invention, itis preferred that the outer circumferential edge of the annular outerportion of the retaining member protrude toward the elastic member, anda side of the elastic member that is in contact with the retainingmember enter into a concave portion between the inner circumferentialedge and the outer circumferential edge formed by the protrusions of theinner circumferential edge and the outer circumferential edge of theretaining member.

According to the above-described configuration, when the elastic memberis deformed as a gap between the cone and the retaining member isnarrowed inside the wellbore, the elastic member can be pressed againstthe retaining member with less load. Accordingly, the wellbore can beplugged with less load.

Further, it is preferred that in the plug according to an aspect of thepresent invention, the retaining member be composed of a degradableresin or a degradable metal.

According to the above-described configuration, the retaining member canbe degraded after the plug is used, making it possible to dispense withthe time and effort necessary for collecting the retaining member afterthe plug is used.

Another aspect of the present invention provides a retaining member usedfor a plug provided at a well during completion and having a function ofplugging a wellbore. A retaining member according to an aspect of thepresent invention is a retaining member having an annular shape, usedfor a plug configured to plug a wellbore provided at a well duringcompletion,

the plug including:

a tubular member having a hollow structure; and

an elastic member with an annular shape attached to an outercircumferential surface of the tubular member, the annular elasticmember being deformable by exerted pressure, wherein

the retaining member is attached, adjacent to the elastic member, ontothe outer circumferential surface of the tubular member, downstream ofwhere a pressure is exerted on the elastic member,

the retaining member being configured to include

an annular inner portion which comes into contact with the outercircumferential surface of the tubular member, and

an annular outer portion having an inner diameter that is equivalent toor larger than the annular inner portion and being movably attached tothe annular inner portion,

the annular outer portion including a face that is to face the elasticmember.

Still another aspect of the present invention provides a method for wellcompletion. The method for well completion according to the stillanother aspect of the present invention uses a plug according to anaspect of the present invention.

The present invention is not limited to the embodiments described above,and may be variously altered within the scope set forth in the claims.That is, embodiments obtained by combining appropriately alteredtechnical means within the scope set forth in the claims are encompassedwithin the technical scope of the present invention.

EXAMPLES

In order to investigate a technical advantage of the downhole plugaccording to another aspect of the present invention, the followingexperiments were conducted.

Example 1 Formation of Downhole Plug (A)

A downhole plug (A) having a configuration described in theabove-described embodiment was prepared as a downhole plug of Example 1.Note that the mandrel, socket, and cones were formed from PGA, the pairof slips, and the pair of fixing members were formed from magnesiumalloy, and the sealing member was formed from polyurethane rubber.

Evaluation of Water-Pressure Test Results

The downhole plug (A) was fixed in a casing by performing an actuation.Next, water was sealed within the casing while heating the inside of thecasing to a temperature of 93° C., then water pressure of 10000 psi(approximately 70 MPa) was exerted on the downhole plug (A) with a pumpto examine whether the downhole plug (A) can retain the water pressurefor not less than 30 minutes. Then, if the downhole plug (A) couldretain the water pressure for not less than 30 minutes, an evaluation of“Pass” is given as acceptable, while if the downhole plug (A) could notretain the water pressure for not less than 30 minutes, an evaluation of“Fail” is given as not acceptable. The results are listed in Table 1.

Evaluation of Anti-Extrusion Effect on Sealing Member

After the water-pressure test was conducted, a sealing member in thedownhole plug (A) was visually confirmed. Then, if no extrusion on thesealing member due to a breakage of the socket or no entry of thesealing member between the socket and the mandrel was observed, anevaluation of “Pass” is given as an anti-extrusion effect on sealingmember was present, while if extrusion or entry of the sealing memberwas observed, an evaluation of “Fail” is given as the anti-extrusioneffect on the sealing member was not present. The results are listed inTable 1.

Reference Example 1 Formation of Downhole Plug (B)

A downhole plug (B) was prepared as a downhole plug of Reference Example1, where the downhole plug (B) has a configuration as in the downholeplug (A) except for using a socket composed of a single member, eventhough the inner circumferential edge and the outer circumferential edgeof the socket protrude toward the sealing member. The water-pressuretest results and the anti-extrusion effect on the sealing member werethen evaluated by the same procedure as in Example 1. The results arelisted in Table 1.

Comparative Example 1

A downhole plug (C) was prepared as a downhole plug of ComparativeExample 1, where the downhole plug (C) has a configuration as in thedownhole plug (A) except for using a socket in which the innercircumferential edge of the socket does not protrude toward the sealingmember as illustrated in FIG. 4. The water-pressure test results and theanti-extrusion effect on the sealing member were then evaluated by thesame procedure as in Example 1. The results are listed in Table 1.

TABLE 1 Actuation Load Water-pressure Suppression of Sealing (kN) testmember Example 1 66 Pass Pass Reference 162 Pass ×(Extrusion Example 1due to breakage of socket Observed) Comparative 98 Fail ×(Entry Example1 between socket and mandrel Observed)

The downhole plugs of Example 1 and Reference Example 1 could retain asealed state under water pressure of 10000 psi for not less than 30minutes. The downhole plug was disassembled to confirm that theextrusion of the sealing member to the inside of the socket had beensuppressed, and it was confirmed that breakage of the mandrel had beensuppressed. However, in Reference Example 1, a breakage of the socketwas confirmed, and a extrusion of the sealing member due to this wasobserved.

On the other hand, in the downhole plug of Comparative Example 1, theseal sate could not be retained under water pressure of 10000 psi fornot less than 30 minutes. The downhole plug was disassembled to confirmthat the sealing member had extruded to the inside of the socket, tothus constrict the mandrel to cause breakage of the mandrel.

INDUSTRIAL APPLICABILITY

The present invention can be used as a plug having a function ofplugging a wellbore provided at a well during completion.

Reference Signs List   1 Mandrel (Tubular member) 2 Sealing member(Elastic member) 3 Socket (Retaining member) 4, 5 Cone 6a, 6b Slip 7a,7b Ring-shaped fixing member 10 Downhole plug (Plug) 31 Socket-innerportion 32 Socket-outer portion 322 Face

1. A plug configured to plug a wellbore, the plug comprising: a tubularmember having a hollow structure; an elastic member having an annularshape and being attached to an outer circumferential surface of thetubular member, the annular elastic member being deformable by anexerted pressure; and a retaining member having an annular shape andbeing adjacent to the elastic member, wherein the retaining memberincludes: an annular inner portion which is in contact with the outercircumferential surface of the tubular member; and an annular outerportion having an inner diameter that is equivalent to or larger thanthat of the annular inner portion and being adjacent to the annularinner portion, the annular outer portion includes a face facing theelastic member, and the annular inner portion includes a face that is incontact with the annular outer portion and that is inclined so as to beaway from the annular elastic member as extending toward an outerperiphery of the plug.
 2. The plug according to claim 1, wherein aninner circumferential edge of the annular inner portion that comes incontact with the tubular member protrudes toward the elastic member. 3.The plug according to claim 2, wherein an outer circumferential edge ofthe annular outer portion of the retaining member protrudes toward theelastic member, and a side of the elastic member that is in contact withthe retaining member enters into a concave portion between the innercircumferential edge of the annular inner portion and the outercircumferential edge of the annular outer portion.
 4. The plug accordingto any one of claim 1, wherein the retaining member is composed of adegradable resin or a degradable metal.
 5. A retaining member having anannular shape, used for a plug configured to plug a wellbore, the plugcomprising: a tubular member having a hollow structure; and an elasticmember having an annular shape and being attached to an outercircumferential surface of the tubular member, the annular elasticmember being deformable by an exerted pressure, wherein the retainingmember is adjacent to the elastic member, the retaining member includes:an annular inner portion configured to be in contact with the outercircumferential surface of the tubular member; and an annular outerportion having an inner diameter that is equivalent to or larger thanthat of the annular inner portion and being adjacent to the annularinner portion, the annular outer portion includes a face configured toface the elastic member, and the annular inner portion includes a facethat is in contact with the annular outer portion and that is inclinedso as to be away from the annular elastic member as extending toward anouter periphery of the plug.
 6. A method for well completion, the methodcomprising using the plug according to claim 1, including: applyingpressure to the plug to cause the elastic member to deform against theretaining member.